networking – Spurious Retr & Dup ACK

We have a server behind a firewall and numerus clients on the road are using an app that connects to the server, but some commands from the app never arrive at the server so we started tracing the traffic on the server. We found out numerus cases like this
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The client sends 2 packets in a row 11994-5, the server send ACK only for the second packet and then we have a Spurious Retr & a Dup ACK. If I understand correctly the server receives the packet 11994 but is not sending an ACK, only an Dup ACK like it didn’t receive it…. How is that possible?